fix: strip _NEXTSEP_ from interpolated pathnames (#84430)
## What
Fixes an issue where internal route normalization markers (`_NEXTSEP_`)
were leaking into compiled URLs for interception routes with adjacent
dynamic parameters.
## Why
When Next.js normalizes route patterns like `/photos/(.):author/:id` for
path-to-regexp validation, it inserts `_NEXTSEP_` as a literal separator
between adjacent parameters. This internal marker was appearing in the
final compiled URLs (e.g., `/photos/(.)_NEXTSEP_next/123` instead of
`/photos/(.)next/123`), exposing implementation details to users.
## How
- Modified `safeCompile` to wrap the path-to-regexp compiler when
normalization is applied
- The wrapper calls `stripNormalizedSeparators` to remove the internal
marker from compiled output
- Added targeted regex pattern that only strips separators after
interception route markers (`)_NEXTSEP_` → `)`)
- Preserved separators in user content that legitimately contains this
string
NAR-431
